http://www.uswitch.com and http://www.moneysavingexpert.com have comparisons for mobile tariffs. Although bear in mind this is a fast moving market and comparisons may not be up to date. EE for instance have just refreshed their sim only tariffs and the comparison sites may need some catching up
Give what you have said about data being your main concern, unless you use your phone for tethering, both 3 and virgin media have unlimited data tariffs that would suit your needs for in the region of £15-£18 a month. If you do need tethering, 3 will allow it but you would be limited to 4GB of tethering a month on their unlimited plans. Virgin don't allow it.
If you do sign up for a sim only somewhere check http://www.topcashback.co.uk or http://www.quidco.com as you can sometimes get cashback for signing up. I got £10.50 the other month for taking out a sim only with O2 for a family member and £70 this week for signing up to EE
